On official sites for more well-known ABDL companies (like Tykables or ABUniverse) there is usually a size chart or size guide available so that you can compare your body measurements to the measurements of the garment(s) they are offering. On market sites like Etsy then generally most reputable sellers are usually kind enough to list off a size guide in the description of most items like adult onesies. You will want to compare the listing or size chart to make sure that the size you are purchasing is not smaller than your measurements. It's been my experience that if I'm unsure about the sizing or material-type then sellers have been very prompt and kind to try to help me by clarifying measurement listings or stretch-ability of some fabrics.
Here is a list of online stores, shops, and common sellers for onesies/bodysuits:
https://www.datecgl.com/linkssharing/br ... ag=onesies